1. Field of the Invention
The present invention relates to a process for forming dicing lines on a wafer, more particularly to a process for forming a dicing line pattern in a wafer region which has not been exposed to light during the formation of an LSI pattern of the wafer, by using a specific reticle for forming dicing lines or zones.
2. Description of the Related Art
In the production of semiconductor devices, reticles or masks are used for forming patterns of an electric circuit, etc., on a wafer by a projection exposure process, (i.e.) a photolithography process in which a pattern on the reticle or mask disposed not in contact with but above or apart from the wafer is projected and transferred onto the wafer surface by exposing the wafer to light irradiated through the reticle, whereby the wafer is patterned in a form defined by the patterns on the reticle and in a desired miniaturization, for example, 1/5 or 1/10. A device pattern formed on a reticle is stepwise repeatedly transferred onto the wafer surface by exposure to light in a "step-and-repeat" manner, and during this exposure, the dicing (or scribe) lines (or zones) are simultaneously transferred onto the wafer. The term "dicing lines" referred to herein means specific linear regions or zones used for dicing or dividing a wafer into individual chips.
FIG. 1 shows a plan view of a conventional reticle for forming an LSI pattern on a wafer, wherein a reticle 1 has a device pattern region 2 such as an electric circuit pattern (represented by an "F"-shaped pattern in the figure) formed in the central portion thereof and a dicing zone 3 having a predetermined width formed squarewise and in contact with the periphery of the device region 2. An alignment pattern 4 or the like is usually formed at a predetermined portion of the dicing zone 3, and a margin pattern 5 for protecting the alignment pattern 4 from exposure is formed at the portion corresponding to the alignment pattern 4. As shown in FIG. 2, the required patterns are formed shot by shot on a wafer by exposure to light in a manner such that patterns of the dicing zone 3 projected on the wafer are partially overlapped or subjected to duplicated or double exposure for each shot.
FIG. 3 shows another typical conventional reticle. In a reticle or mask 6, a dicing zone 3 is formed in a "L" shape and in contact with the periphery of a device pattern region 2 at the adjoining two sides thereof. By using this mask 6, the required patterns are formed on a wafer by shot-by-shot exposures to light in a manner such that the mask patterns projected on the wafer by respective exposures are not overlapped but are side by side and in contact with each other, as shown in FIG. 4.
Accordingly, upon exposure by using such reticles, the device pattern region and the dicing zone are simultaneously exposed so that the dicing zone 3 for dicing of a wafer by a dicer or dicing machine is also formed in contact with the periphery of the device pattern region 2 on the wafer.
Until recently, the LSI pattern formed on a reticle has been relatively smaller in size corresponding to a relatively smaller chip size, so that the whole surface of a wafer can be subjected to exposure to form dicing zones thereover. The chip size, however, is becoming larger and larger and some exposure apparatuses are now unable to effectively cover the circumferential region of a wafer. Also, the circumferential wafer region cannot be used essentially as an effective wafer region for providing sound chips due to unavoidable crystal defects which occur during the production and handling of wafer. Accordingly, the circumferential wafer region is not subjected to exposure and, in some instances, dicing lines are not completely formed in the circumferential region. Referring to FIG. 9, the pattern exposure is not carried out in the non-hatched region of a wafer 14, with the result that the aluminum layer for wiring, etc., is retained over the whole surface of the region. Therefore, when a reticle 1 of FIG. 1 having a square-shaped dicing zone 3 is used, the circumferential region cannot be subjected to a double exposure, with the result that a margin pattern 5 is retained. On the other hand, when a reticle 6 of FIG. 3 having an "L"-shaped dicing zone 3 is used, a region in which dicing lines are not formed is retained on one side of a wafer in accordance with the direction of a shot-by-shot exposure progress.
Also in the central device pattern region of a wafer, a region in which dicing line patterns are not formed is retained when an insertion pattern such as a monitor pattern is formed by using an insertion pattern reticle different from a device pattern reticle.
FIGS. 5A, 5B, and 5C show a device pattern reticle 51, and insertion pattern reticles 501 and 511, respectively, which all are the same size. The device pattern reticle 51 has a central device pattern region 52 (the pattern represented by "F, ") and a dicing zone 53 in contact with the periphery of the device pattern region 52. The insertion pattern reticles 501 and 511 have respective insertion pattern regions 502 and 512 (the patterns represented by "x" and "y") which are smaller than the device pattern region 52 of the reticle 51, but have no dicing zone.
On a wafer partially shown in FIG. 6A, the device patterns "F," in the regions 62 and the dicing lines 63 are formed by using the reticle 51 in a stepand-repeat manner, during which the portion including regions 602, 603, and 612 is skipped, i.e., this portion is not subjected to exposure, and then the insertion patterns "x" and "y" are formed at sites 602 and 612, respectively.
In the above pattern formation processes, the region 603 between the regions 602 and 612 in which the insertion patterns "x" and "y" are formed is not subjected to exposure and the required dicing line is not formed for this region 603, as shown by imaginary broken lines.
In some cases, a margin pattern is retained in the central device pattern region of a wafer, as typically shown in FIG. 7A. The device patterns "F" in the regions 72 and the dicing lines 73 are formed by using the reticle 1 of FIG. 1 and an insertion pattern "x" of FIG. 5B, which is smaller than the device pattern "F", is formed at the site 702. During this formation process, a margin pattern 75 formed together with the pattern "F" of the left hand side region 72' is not subjected to exposure and is retained.
As mentioned above, regions in which dicing lines are not formed are retained in the circumferential region and, in some cases, the central device region of a wafer. The presence of such retained unexposed regions in which dicing lines are not formed or are incompletely formed causes a problem in that, upon dicing of a wafer into LSI chips, the retained unnecessary layer of aluminum, etc., damages the cutting edge of a dicing machine, and the aluminum dust, etc., is scattered and causes contamination of the wafer, the cut LSI chip products, and the dicing machine. The unnecessary pattern layers also cause dust during the later cleaning process, in which such layers may be exfoliated.